Handle scale factor change on web-sys backend (#1690)

* Change web backend `event_handler` to without  'static lifetime

* Refactor web runner and fix ControlFlow::Exit not being sticky

* Impl. scaling change event for web-sys backend

* Improve `dpi` docs regarding the web backend

* Add changes to changelog

* Update features.md

src/platform_impl/web/mod.rs

@@ -46,3 +46,9 @@ pub use self::window::{
 };
 
 pub(crate) use crate::icon::NoIcon as PlatformIcon;
+
+#[derive(Clone, Copy)]
+pub(crate) struct ScaleChangeArgs {
+    old_scale: f64,
+    new_scale: f64,
+}